128 HOW CROPS GROW 
H;0; 50,, + Fe = Fe 0,50; a. 
If, now, lime-water or potash-lye be added to the sila 
tion of iron thus obtained, a white or greenish-white pre 
cipitate separates, which is a hydrated protoxide of iron, 
(Fe O,2 H,O). This precipitate rapidly absorbs oxygen 
from the air, becoming black and finally brown. The 
anhydrous protoxide of iron is black. Carbonate of 
protoxide of iron is of frequent occurrence as a mineral 
(spathic iron), and exists dissolved in many mineral wa- 
ters, especially in the so-called chalybeates. 
Sesquioxide of Tron,* Fe, O,, 160.—When protoxide 
of iron is exposed to the air, it acquires a brown color from 
union with more oxygen, and becomes hydrated sesqui- 
oxide. The yellow or brown rust which forms on surfaces 
of metallic iron when exposed to moist air is the same 
body. Iron in the form of sesquioxide is found in the ashes 
of all agricultural plants, the other oxides of iron passing 
into this when exposed to air at high temperatures. It is 
found in immense beds in the earth, and is an important 
ore, (specular iron, hematite). It dissolves in acids, 
forming sesquisalts of iron, which have a yellow color. 
MacGnetic OxiveE or [Ron, Fes Oy, or FeO, Fe. Os, is a combination 
of the two oxides above mentioned. It is black, and is strongly attract- 
ed by the magnet. It constitutes, in fact, the native magnet, or load- 
stone, and is a valuable ore of iron. 
MANGANESE AND ITS COMPOUNDS. 
Manganese, Mn, 55.—Metallic manganese is difficult te 
procure in the free state, and much resembles iron. Its 
oxides which concern the agriculturist are analogous to 
those of iron just noticed. 
Protoxide of Manganese, Mn O, 71, has an olive 
green color. It is the base of all the usually occurring 
* The prefix sesqui (one and a half) is applied to tt ose oxides in which the 
ratio of metal to oxygen is as one to one and a half, or, what is the sane, ng 
two to three. The above compound is also called peroxide of iron. 
